The present invention relates to an improved electro-acoustic transducer element, and more particularly relates to an improvement in or modification of an electro-acoustic transducer element utilizing the vibration mode in the thickness direction of a polymeric piezoelectric film as disclosed in Japanese Patent Publication No. 78/26799 (TOKKOSHO 53-26799). The present electro-acoustic transducer element is used for transmission and/or conversion of ultrasonic waves.
As a substitute for the conventional inorganic piezoelectric material, polymeric piezoelectric material may be advantageously used for ultrasonic vibrators in the field of diagnostics and detection of internal defects in various articles. Advantages are its easy production of large-sized films, easiness in treatment and fine fit to curved surfaces.
The acoustic impedance of a polymeric piezoelectric material is far lower than that of inorganic piezoelectric materials and very close to those of water, organisms and general organic materials. Thus, the polymeric piezoelectric material functions as an excellent transmitter and receiver for ultrasonic waves which travel through these objects.
However, the use of polymeric piezoelectric films in the construction of an ultrasonic transducer is, in practice, accompanied with various problems.
In the case of ultrasonic devices used for diagnostics and/or detection of internal defects, ultrasonic waves are mostly used with frequencies in the range from 1 to 10 MHz.
It is well known that, in order to obtain high transmission efficiency, the resonant frequency of the vibrator has to match the frequency of the ultrasonic wave to be used for the process. In other words, the thickness of the piezoelectric film has to be chosen in accordance with the frequency of the ultrasonic wave to be used for the intended process.
In the case of polyvinylidene fluoride which is a typical polymeric piezoelectric material, its frequency constant (F).times.(T) is nearly equal to 115 KHz.multidot.cm, (F) being the resonant frequency of a free thickness vibrator and (T) being the thickness of the film. In order to obtain high efficiency in transmission of an ultrasonic wave of 2.5 MHz frequency which is commonly used for diagnostic purposes, it is required for the film to have a thickness of 460 .mu.m (micrometer) for a half wave drive, and 230 .mu.m for a quarter wave drive.
A potential of about 10.sup.6 V/cm is needed for polarization of polymer to provide for piezoelectricity. Polarization of a polymer film of a large thickness is often accompanied with trouble such as aerial discharge, thereby disabling easy preparation of a thick polymer piezoelectric film. The conventionally available thickness under the present technology is typically 100 .mu.m or smaller. This is the first disadvantage of the conventional art.
In the production of a polymeric piezoelectric film, it is very difficult to optimumly control the process in order to provide the resultant film with a thickness well suited for transmission of the ultrasonic wave of a desired frequency. Such a polymer piezoelectric film is in most cases obtained by polarization of a material film after drawing. Depending on the process conditions in drawing and heat treatment, thickness of the resultant film varies greatly. Quite unlike the inorganic piezoelectric material, it is extremely troublesome and, consequently, almost infeasible to adjust the thickness of a polymer piezoelectric film by means of polishing or griding. This is the second disadvantage of the conventional art.
Dielectric constant of a polymer piezoelectric film is in general not so high as that of the inorganic piezoelectric material such as PZT. Therefore, increase in thickness of the film causes reduction in electric capacity. As a resultant, an increased electric impedance of the vibrator does not well match that of the electric power source, thereby blocking smooth supply of energy to the vibrator from the electric power source. This is the third disadvantage of the prior art.